Diffraction grating and system for the formation of color components

ABSTRACT

A device for spacially separating specific spectral regions, preferably of color components from a wideband spectrum which is actively and/or passively radiated by objects. The spectral regions, or color components, are derived from the diffraction orders of a diffraction grating (phase grating), which is disposed in the pupil of an imaging lens and whose groove profile consists of several steps, which produce path length differences which are integral multiples of a specific wavelength.

This is a continuation of application Ser. No. 838,632, filed Oct. 3,1977 now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to a device and a system for spacially separatingspecific spectral regions, preferably color components of a wide-bandspectrum, multichromatic light, which is actively and/or passivelyradiated by objects. Various devices for the production of colorcomponents are known. For example, in the field of optics simpleabsorption color filters may be employed, which however have thedisadvantage of a low luminous efficiency. In, for example, colortelevision camera applications this disadvantage is eliminated by theuse of so-called dichroic mirrors (color-selective, wide bandinterference filters). However, such mirrors are very complicated tomanufacture and hence expensive.

SUMMARY OF THE INVENTION

It is an object of the invention to provide means for deriving colorcomponents, which means have a high luminous efficiency and are simpleto manufacture.

This problem is solved in that the spectral regions, or colorcomponents, are derived from the diffraction orders of a diffractiongrating (phase grating) which is arranged in the pupil of an imaginglens and whose groove profile consists of a plurality of steps whichproduce differences in optical path length which equal integralmultiples of a specific wavelength.

Such diffraction gratings have substantially the same luminousefficiency as dichroic mirrors, but can be manufactured considerablycheaper in large quantities using pressing methods.

Preferred fields of application are film scanning, color television andcolor facsimile. However, the range of application is not limited to thevisible spectral range, but may be easily extended up to the near andfar infra-red ranges and beyond these.

BRIEF DESCRIPTION OF THE DRAWINGS

An example and the operation of the diffraction grating are described inmore detail with reference to the drawing. In the drawing:

FIG. 1 shows a device according to the invention in an opticalarrangement for deriving color components,

FIG. 2 shows an example of a diffraction grating according to theinvention with a stepped groove profile,

FIG. 3 shows the spectral distribution of the light among thediffraction orders when using diffraction gratings with a groove profilein accordance with FIG. 2,

FIG. 4 shows an optical arrangement using a diffraction grating for theelimination of color dispersion in the diffracted orders.

FIG. 5 shows an alternative embodiment using a prism to eliminate colordispersion in the diffracted orders.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The operation of the new diffraction grating (phase grating) is based onthe fact that the spectral distribution of the light among thediffraction orders greatly depends on the groove profile of saidgratings. This groove profile may for example be adapted so that thezeroth diffraction order, i.e. the non-diffracted ray, appears in thegreen light region, one first diffraction order (for example, the +1order) in the blue region, and the other first diffraction order (forexample, the -1 order) in the red region. If such a grating is disposedin the pupil of an imaging lense, the corresponding color components inblue, green, and red are obtained side by side instead of as a normalimage.

Without diffraction grating G the optical arrangement in accordance withFIG. 1 would produce the normal color image of the object Ob at thelocation of the color component g. By means of the diffraction gratingsecondary images corresponding to the diffraction orders are obtained atthe locations thereof, which in the case of an adequate spacing betweenthe diffraction orders relative to the size of the images appearseparately adjacent each other in the image plane. The groove profile ofthe diffraction grating (phase grating) is now adapted so that, forexample, at the location of the central normal image a green imagecomponent appears, and at the location of the two first orders a redimage component and a blue image component appear, respectively.

This can for example may be achieved with a groove profile in accordancewith FIG. 2, which consists of a stepped structure, formed from adielectric plate having the refractive index n. The plate issubstantially planar and the plane of the plate can be defined by twoorthogonal axes therein. The geometrical height, or thickness, varies asa repetitive, stepped function of location on the plate, preferably as afunction along only one coordinate axis, and is selected so that thedifference in optical pathlength (n-1)d produced in it (relative tovacuum or air) is an integral multiple of a selected wavelength λ_(g),λ_(g) here representing the central wavelength of the light in the greenimage component:

    (n-1)d.sub.1 =K.sub.1 ·λ.sub.g ; (n-1)d.sub.2 =K.sub.2 ·λ.sub.g                                  (1)

where d₁ and d₂ are the geometrical thicknesses of the two steps in eachstaircase portion in accordance with FIG. 2, and K₁ and K₂ are integers.Thus, the plate is provided with a series of parallel grooves.

Light of the wavelength λ_(g) is then not diffracted by the diffractiongrating, whereas light of any different wavelength λ is diffracted fromthe direction perpendicular to the plane of the grating depending on λ.By the formation of asymmetrical steps, in combination with a suitablechoice of values K₁ and K₂, it can be ensured that blue light is mainlydiffracted in one first order and at the same time red light is mainlydiffracted in another first order. In FIG. 2 these values have beenselected so that K₁ =2 and K₂ =4.

The spectral light distribution among the direct beam and the two firstdiffraction orders associated with the groove profile of FIG. 2 is shownin FIG. 3. For the calculation of the three curves it has been assumedthat the refractive index for the spectral range under consideration,from approx. 400 to 700μ, does not depend on the wavelength. For thecurves in FIG. 3 λ_(g) =525μ and the maxima of the intensitydistribution in the two other color components (blue, red) are situatedat 459μ and 656μ respectively. At the locations of maximum intensity inone color component the two other color components have zero intensity.

FIGS. 2 and 3 illustrate the situation for a particularly simple exampleof a groove profile, where moreover the refractive index n is assumed tobe constant. Other groove forms may be obtained by other suitablechoices of the values K₁ and K₂. Furthermore, the groove profiles maycomprise more than two stages of steps (in addition to the basic stage),in which case again other suitable combinations of the values K₁, K₂, K₃etc. can be selected. Moreover, the refractive index n and its variationas a function of the wavelength of light λ may be adjusted betweencertain limits by the selection of suitable dielectrics.

Consequently, there are many groove profiles which are suitable for theformation of color component images by diffraction gratings. They differin producing different spectral distributions (spectral bands) for thecolor components (example of FIG. 3), in which diffraction orders higherthan the first order may be produced and in which more than three colorcomponents may be produced.

Of special significance are stepped groove profiles (see FIG. 2) with asmall number of steps and values for K₁, K₂, K₃ etc, which are not toohigh. Stepped groove profiles with steps which produce opticalpathlengths differences equal to integral multiples of a specificwavelength, have the special advantage that the color componentcorresponding to this wavelength is produced on the optical axis as acentral undiffracted zero-order image. This central image then exhibitsno color dispersion.

Depending on the spectral width of the color component the non-centralimages exhibit varying degrees of color dispersion, i.e. they areblurred in one direction (the direction of splitting by the grating),thereby having a lower resolution than in the other direction. Forcertain applications this reduced resolution owing to dispersion istolerable, to an extent. In color television the color signals for redand blue can be transmitted with reduced bandwidth, which corresponds toa reduced resolution in the corresponding color components. For filmscanning and color facsimile only one dimensional images are involvedright from the beginning (owing to the line scanning), so that a certaindispersion transversely to the line will have no adverse effect.

In applications in which color dispersion is not tolerable, thisdispersion should and can be eliminated by secondary imaging of therelevant color components with an optical dispersion-compensatingcomponent. A possibility for this is shown in FIG. 4: the dispersionappearing in the original color component (for example r₁ in FIG. 4) iscompensated through imaging by means of a grating G'. This grating may,for example, be a diffraction grating in accordance with the invention.However, in this case it is advantageous to use a grating whose grooveprofile is optimum for minimizing the loss of light in the relevantcolor component.

An alternative which gives at least an approximate compensation of thedispersion is the use of a prism P instead of the grating G' (FIG. 5).In each case a geometrical separation of the images formed in accordancewith FIG. 1 is necessary, which can be achieved without loss of light,for example by means of mirrors. The dispersion in the non-centralimages can then be eliminated for all these images at the same time inone optical path.

What is claimed is:
 1. A diffraction grating, for dividingmultichromatic light passing through the grating into spacially separatespectral regions, comprising a substantially planar dielectric platehaving a thickness which varies as a repetitive, stepped function oflocation on the plate, said function being chosen to produce opticalpath length differences, in said light, which are integral multiples ofa selected wavelength.
 2. A diffraction grating, as claimed in claim 1,wherein the repetitive, stepped function is also asymmetric.
 3. Adiffraction grating, as claimed in claim 2, wherein the plane of theplate has two orthogonal coordinate axes therein, and the thickness ofthe plate varies as a function along only one coordinate axis of theplate, and wherein the function is asymmetric about any planeperpendicular to the one coordinate axis, whereby the plate has a seriesof grooves therein running parallel to the direction of the othercoordinate axis of the plate.
 4. A diffraction grating, as claimed inclaim 3, wherein the selected wavelength is in the green spectrum ofphysical light, whereby a green spectral region is formed as thecentral, undiffracted order.
 5. A diffraction grating, as claimed inclaim 4, wherein two first order spectral regions are produced, onebeing a blue region and the other being a red region.
 6. A diffractiongrating, as claimed in claim 3, wherein the selected wavelength is inthe infra-red spectrum.
 7. A diffraction grating, as claimed in claim 3,4, or 5, wherein the thickness of the plate varies in three steps whichproduce path length differences of 2 and 4 wavelengths and wherein theplate is substantially non-reflective.
 8. An optical system, fordividing multichromatic light passing through the system into spaciallyseparate spectral regions, comprising:a substantially planar dielectricplate, the plane of the plate having two orthogonal coordinate axestherein, said plate having a thickness which varies as a repetitive,stepped function along only one coordinate axis of the plate, saidfunction being asymmetric about any plane perpendicular to the onecoordinate axis, said function being chosen to produce optical pathlength differences, in said light, which are integral multiples of aselected wavelength, said dielectric plate being disposed in the pupilof an imaging lens, whereby a plurality of spacially separated spectralregions are formed from said light passing through said plate; and anoptical dispersion-compensating component, arranged in at least onespectral region, for eliminating dispersion of the wavelength componentsof said spectral region.
 9. An optical system, as claimed in claim 8,wherein the optical component comprises a prism disposed behind a secondimaging lens.
 10. An optical system, as claimed in claim 8, wherein theoptical component comprises a second dielectric plate, having athickness which varies as a repetitive, stepped function of location onthe plate, said function being chosen to produce optical path lengthdifferences which are integral multiples of the average wavelength ofthe one spectral region, said plate being disposed behind a secondimaging lens.
 11. A diffraction grating, for dividing multichromaticlight passing through the grating into spacially separate spectralregions, comprising a substantially planar dielectric plate having aplurality of identical staircase portions in an asymmetric, repetitivepattern, each portion having at least two steps whose thicknesses aredifferent integral multiples of a selected wavelength, said plate beingsubstantially non-reflective.